1. Field of the Invention
The present invention relates to a packaging material capable of preserving freshness of foodstuffs, above all perishables, that may be formed into boxes, bags, wrapping paper, and the like and to a method of fixing the freshness preservative substance onto the packaging material by means of physical and/or chemical processing to preserve freshness as long as possible.
2. Description of the Prior Art
A known wrapping film is manufactured through the process of dispersing minute particles into the film material by crushing and comminuting a solid substance which emits far-infrared rays of about 3-14 .mu.m at room temperature as, for example, far-infrared radioactive ceramics. Products such as packing bags made of this film and packing boxes covered by this film are available in the market. Vegetables, meat, fish and other foodstuffs in fresh condition wrapped by this film are believed to be preserved in freshness longer as packed therein by virtue of absorption of the far-infrared rays radiated from the minute particles in the film.
Further, such corrugated cardboard boxes are available in the market having interiors covered by cristobalite-containing thin paper that is manufactured by mixing the paper material with minute particles of cristobalite capable of gas adsorption. When packed in these corrugated cardboard boxes, the freshness of vegetables and fruit are preserved longer by virtue of the cristobalite minute particles which absorb ethylene gas emitted from the fresh vegetables and fruit themselves. The mechanism is that when the fresh vegetables and fruit are put into contact with the ethylene gas that they, themselves emit, the gas quickens their breathing pace and thus more energy is exhausted and maturity is accelerated resulting in earlier decay. On the other hand, when the ethylene gas is adsorbed by an adsorptive substance and kept away from the vegetables and fruit, such deleterious effects are eliminated and freshness is better preserved.
When a substance which is oxygen-reactive at room temperature, such as iron, aluminum, copper and ascorbic acid, is sealed up by a packaging material together with foodstuffs, the decay of the foodstuffs is suppressed as said substance takes up oxygen inside and thus results in longer preservation of the freshness. However, such wrapping material has never been proposed.
In order to disperse the far-infrared radioactive minute particles into the polyethylene film, it is necessary to mix the minute particles with the resin liquid during the manufacturing of the film, however, achievement of an even and uniform dispersion of the minute particles overall is very difficult and, further, the manufacturing of the film itself becomes time-consuming, thus the manufacturing cost is inclined to be increased because of the presence of the minute particles. Moreover, when the film is used for packing boxes, it should always be placed over the inside surface and, as a result, processing cost may be further increased.
The cristobalite-containing thin paper, on the other hand, requires another sort of bothersome processing procedure of paper making with the cristobalite minute particles, which is timeconsuming. Further the cristobalite-containing thin paper is required to be applied to boxes over the inside surface of boxes, which further increases the manufacturing cost. Further, this cristobalite-containing thin paper suffers from the defect of the minute particles falling off.